1. Field of the Invention
The present invention relates to a method of writing into an electrically alterable non-volatile semiconductor memory. More specifically, the present invention concerns a method of selectively writing any one of more than two values into each memory cell.
2. Description of the Related Art
An electrically writable and erasable non-volatile semiconductor memory is known as an EEPROM. In particular, there is such an EEPROM capable of wholly erasing data written therein, which is referred to as a flash EEPROM. Each of memory cells of this flash EEPROM employs a MOS transistor including a drain region and a source region, which are fabricated on one surface of a substrate and are mutually separated from each other, and a channel region formed therebetween. The MOS transistor further includes a tunnel insulating film, a floating gate electrode, an interlayer insulating film, and a control electrode, which are successively formed on this channel region.
To write data into such a memory cell, electron charges are injected via the tunnel insulating film into the floating gate, thereby causing the memory cell to take a first state indicating, for instance, data "1" where a threshold voltage of the memory cell is set to a high value, and conversely, data "0" is represented by a second state of the memory cell where the threshold voltage of the memory cell is set to a low value by removing the electron charges from the floating gate. This stored data is read out by sensing whether the memory cell is set into the first state, or the second state, by applying to the control gate electrode a voltage lower than the above-explained higher threshold voltage value and higher than the lower threshold voltage value.
The above-described conventional EEPROM can store only data having two different values, since the respective memory cells are selectively set to one of the first and second state, resulting in small memory capacities. To allow a recent trend of increasing data amounts, it is desired to develop an EEPROM having a large memory capacity with a small number of memory cells.
To this end, the inventors have proposed a method of selectively writing 4-value data into each of the memory cells of EEPROM in "The Proposal of Multi-bit type Flash Memory", at the 53rd Autumn Meeting, 1992, The Japan Society of Applied Physics, 17a-ZS-9. However, this proposal merely described the very basic idea. Thus, there are many difficulties to practically realize this type of flash memory.